Post-translational modification of membrane proteins with lipids appears to be ubiquitous in prokaryotic cells. Bacterial lipoproteins share the common structural feature of a consensus sequence known as lipobox, i.e., [LVI][ASTVI][GAS]C, at the processing site. See, e.g., von Heijne, Protein Eng 2: 531-534 (1989). Once they are modified with the lipid moieties at their N-termini, they are anchored to the cell surface and function as structural proteins (e.g., murein lipoprotein) or catalytic proteins (membrane-bound enzyme or transport proteins). See, e.g., Hayashi and Wu, J Bioenerg Biomembr 22: 451-471 (1990). The biosynthesis of lipoproteins in E. coli has been studied in detail using Braun's lipoprotein. See, e.g., Braun and Rehn, Eur J Biochem 10: 426-438 (1969). The protein is first synthesized as an unmodified prolipoprotein and then modified by the prolipoprotein diacylglyceryl transferase (Lgt). Lgt catalyzes the transfer of the diacylglyceryl moiety from phosphatidylglycerol to the thiol group of the conserved cysteine. The lipidation signal sequence of diacylglyceryl prolipoprotein is subsequently cleaved at the modified cysteine by prolipoprotein signal peptidase (LspA). Phospholipids-apolipoprotein N-acyltransferase (Lnt) catalyzes the final modification by adding a fatty acid at the N-terminal cysteine to form the mature lipoprotein.
As most of the lipoproteins are located on the bacterial cell surface, they are readily exposed to the host's immune system. The cysteine-linked diacyl lipid moiety of lipoprotein is recognized as a danger signal by the immune system. Lipoproteins are thus critical antigens for protective immunity. Application of recombinant lipoproteins as vaccine candidates has been limited by the expression level, lipid modification, and down-stream processing of the recombinant lipoproteins. Thus, there is a need for an improved method of expressing lipoproteins.